This invention relates generally to a lamp ballast, and more particularly to an electronic ballast for operating two or more pre-heated, fluorescent lamps in parallel.
Conventional rapid start ballasts for powering two or more pre-heated, fluorescent lamps typically operate the lamps in series with a starting capacitor across all but one lamp. Use of a starting capacitor generally results in an increase in glow current when starting the lamp. This increase in glow current frequently lowers the life expectancy of the lamp.
Conventional instant start ballasts for powering two or more fluorescent lamps typically operate the lamps in parallel. Operating conditions for each lamp are dependent on the operation of every other lamp. When any one of the parallel connected lamps fails to operate, there is a change in lamp load. A mismatch in the power outputted by the ballast and the lamp load results. Operation of each parallel connected lamp remaining in operation is adversely affected. In other words, the lamps do not operate independently of one another other.
A conventional lamp ballast often drives its inverter at two different frequencies, that is, at a resonant frequency during pre-ignition of the lamp load and at a steady-state operating frequency. Sensing circuitry to determine when to switch from the resonant frequency to the steady state operating frequency is often required.
The resonant frequency is typically based on the series combination of an inductor L and a capacitor C. It is particularly difficult to operate at or near the resonant frequency of the series L-C circuit before lamp ignition inasmuch as unsafe, high voltages and current levels can occur (i.e. above the maximum ratings of one or more ballast circuit components). By operating below resonance during pre-ignition of the lamp load, capacitive switching of the inverter can easily occur producing high switching losses. Additional circuitry is therefore required to prevent the switching frequency of the inverter from operating below the series L-C circuit resonant frequency during pre-ignition of the lamp load.
Accordingly, it is desirable to provide a lamp ballast for powering two or more pre-heated, fluorescent lamps. The lamps should be operated independently of each other. Reduction in lamp life arising from glow currents should be minimized. The ballast should have safe open circuit (i.e., pre-ignition) voltage and current levels, with relatively low switching losses. The improved lamp ballast should operate at single frequency which is well below the resonant frequency of the series L-C circuit.